1. Field of the Invention
The present invention relates to balance-to-unbalances (baluns), and, more particularly, to a multi bandwidth balun and a circuit structure thereof.
2. Description of Related Art
With the advance of communication technology, nowadays a wireless communication system requires a radio frequency (RF) device operated under multiple frequency band. For example, a balun is an essential circuit element in multi bandwidth communication system and multifunction system. The balun converts the signal from an unbalance state into a balance state between an unbalanced port connected to an antenna and a balanced port connected to an impedance load, and matches the impedances. In general, the balun is constituted by two opposite coils with the same polarity.
FIGS. 1A and 1B illustrate a circuit diagram and a top view of the circuit structure thereof of a balun 1 according to the prior art, respectively.
As shown in FIGS. 1A and 1B, the balun 1 is achieved via forming a balun structure operated in two RF bandwidths on a semiconductor base layer by a semiconductor manufacturing technique. That is, the circuit structure 1′ is disposed on the surface of a base layer 1a having a ground port, and the circuit structure 1′ has a plurality of conductive pads 14.
The balun 1 comprises two main signal ports 17a and 17b, a first main inductor 10a electrically connected to the main signal port 17a, a second main inductor 10b electrically connected to the main signal port 17b, two additional conductors 170 connected in parallel to the first main inductor 10a and the second main inductor 10b, respectively, a first inductor 11a electrically connected to the first main inductor 10a to constitute a first conversion circuit 15a, a second inductor 11b electrically connected to the second main inductor 10b to constitute a second conversion circuit 15b, a first capacitor 12a connected in parallel to the first conversion circuit 15a, a second capacitor 12b connected in parallel to the second conversion circuit 15b, two first signal ports 18a and 18b electrically connected to the first capacitor 12a, and two second signal ports 19a and 19b electrically connected to the second capacitor 12b. 
The main signal ports 17a and 17b are unbalanced signal transmission ports. The first signal ports 18a and 18b and the second signal ports 19a and 19b are balanced signal transmission ports.
However, in the multi bandwidth balun 1 according to the prior art, due to the design of two sets of electrical matching network, the circuit structure 1′ has more elements. The increased number of unbalanced signal transmission ports results in the higher cost of material and manufacturing, and additional switch circuits are required to achieve the function of the balun 1.
Moreover, the circuit structure 1′ is formed on the same surface of a base layer 1a, such that the circuit structure 1′ takes a great portion of the area of the surface of the base layer 1a, and thereby can hardly reduce the length and width off a semiconductor element. In particular, forming two main signal ports 17a and 17b makes the circuit arrangement area of the semiconductor element even harder to be reduced. Therefore, electronic products do not meet the requirement of minimization.
Thus, how to overcome various problems in the prior art is substantially an issue desired to be solved.